A practice of using cement in the oil industry began around 1903 in California in an attempt to stop water from flowing into the oil well, and oil and gas from entering the waterways (aquifers). In those early years, cement was hand mixed and run into a dump baler to the spot needing to be plugged. Pumping cement, which would be mixed with water, down a well was soon also recognized as beneficial to encase the well and achieve a safer and more efficient operation of the drilling process. A forerunner of the modern two (2)-plug method was first used in 1910. The two (2) plugs minimize mud contact with the cement. Although both mechanical and chemical improvements have been made in a cementing process, the original plug concept is still valid today.
Erle Halliburton cemented a well in Oklahoma's Hewitt Field in 1920. The dry blending of oilfield cements is attained by many means, but the most prevalent remains the pneumatic transfer of the individual constituent parts of cements and additives, which are moved from pressure vessel to pressure vessel. This remains very similar to what was developed in the 1920's. These moves are often layered or “pancaked” to affect somewhat of a blend when then transferring to the next in a series of tanks. This type of mixing is referred to as “moves”. Moves result in a very unscientific and haphazard blending methodology, as the differing specific gravities and molecular makeup of the varies constituent materials including various cements, bulk powder additives and granulated minerals and chemicals, are not easily comingled. Cement blends produced in this manner are highly dependent on the experience and attention of the blend plant operator. Numerous problems have been encountered with variations in such un-uniform blending which results in the ununiformed blends needing to be “spiked” or modification of the cement blends at the well site prior to mixing with water to get a properly performing and more complete mixture. A poor cement blend mixture can cause many problems including poor set strength, inadequate cement bond, blowouts, poor formation fracing or lack of mud displacement, which in the least presents environmental hazards, and losses in productivity, and in the worst cases can result in severe injury and loss of life due to blowouts and resulting explosions and fire.
Because of the inconsistencies in the mix cement product, many oilfield operators have gone to “pod” or “batch mixing”. In the pod or batch mixing, all of the ingredients for the cement are put inside of a mixer and stirred together. This mix of blended cement is taken either in a slurry form or a powder form to the wellhead. At the wellhead, if it's in the powder form, water is added as the slurry is injected into the well. The using of pod or batch plants solved to some degree the cementing problems at shallow depths.
However, over the years, many different types of cements have been developed. The American Petroleum Institute recognizes Class A through Class J of different types of cements. When deciding upon cement job not only does a type of cement have to be selected, but so does the various additives. Many different additives have developed over the years. Oil wells have gotten deeper and deeper, and in recent years drilling is both vertical and horizontal, so the cementing occurs at higher pressures and higher temperatures, and the correct cement blend or mixture becomes more and more critical. Each well service company provides its own particular blend or “recipe” for their cement jobs, especially cement used at depths of 10,000 feet or more and are expected by well owners to provide a high level of quality assurance of high-performing well encasement with the pre-blended cements. The invention allows the utmost in quality assurance to the well service contractor, the well owner, landowner, and the community as a whole, while protecting the well rig workers and the environment.
For the blends used at high pressures and temperatures, it becomes important to completely mix (1) large volumes by weight items in the pre-blended dry cements, (2) intermediate volumes by weight of some items in the pre-blended dry cements, and (3) small volumes by weight of additives in the pre-blended cements. All of these must be perfectly pre-blended together to give the ideal cement blend at the well site prior to mixing with water or other fluids. If the ideal dry cement pre-blend is not reached and the cements and additives are not properly applied, blowouts can occur such as the BP Petroleum blowout that occurred in the Gulf of Mexico in 2010. Since the catastrophic BP blowout, more and more attention has been given to the accuracy of the pre-blend of cement being used, especially in deep wells, and in fracturing of wells, and also on offshore drilling.
In an attempt to solve the problem of inadequate pre-blending of oilfield cement, many of the larger companies have developed their own system or techniques. For example, Schlumberger Technology Corporation in U.S. Pat. No. 7,464,757, shows a batch mixing facility to deliver homogenized mixing slurry to a well pumping system, but this mixing can only affect the water/fluid-cement ratios and homogenize the mixture with the fluid, and does nothing to affect the imbalance of poorly pre-blended dry cements delivered to the well mixer out of specification of the recipes and safety requirements.
One of the problems with the prior systems is when water is added to the cement mixture, the resulting slurry can only be as good as the dry pre-blend, which at present is most unscientific and haphazard resulting in varying quality and unknown performance of the well encasement.